Crystal orientation mechanism of ZnTe epilayers formed on different orientations of sapphire substrates by molecular beam epitaxy

Abstract

The electrooptic effect in ZnTe has recently attracted research attention, and various device structures using ZnTe have been explored. For application to practical terahertz wave detector devices based on ZnTe thin films, sapphire substrates are preferred because they enable the optical path alignment to be simplified. ZnTe/sapphire heterostructures were focused upon, and ZnTe epilayers were prepared on highly mismatched sapphire substrates by molecular beam epitaxy. Epitaxial relationships between the ZnTe thin films and the sapphire substrates with their various orientations were investigated using an X-ray diffraction pole figure method. (0001) c-plane, (1-102) r-plane, (1-100) m-plane, and (11-20) a-plane oriented sapphire substrates were used in this study. The epitaxial relationship between ZnTe and c-plane sapphire was found to be (111) ZnTe//(0001) sapphire with an in-plane orientation relationship of [−211] ZnTe//[1-100] sapphire. It was found that the (211)-plane ZnTe layer was grown on the m-plane of the sapphire substrates, and the (100)-plane ZnTe layer was grown on the r-plane sapphire. When the sapphire substrates were inclined from the c-plane towards the m-axis direction, the orientation of the ZnTe thin films was then tilted from the (111)-plane to the (211)-plane. The c-plane of the sapphire substrates governs the formation of the (111) ZnTe domain and the ZnTe epilayer orientation. These crystallographic features were also related to the atom arrangements of ZnTe and sapphire.

ZnTe layers on R ‐ and S ‐plane sapphire substrates

“…The XRD spectra of the ZnTe layer grown on the R-plane and the S-plane sapphire exhibited a peak originating from the sapphire substrate and a dominating peak originated from the (111) plane of ZnTe. The diffraction spectra of ZnTe layers on those substrates were similar to that obtained from the layer grown on the c-plane sapphire [14,[16][17][18]. Since the θ-2θ profile is not powerful enough to clarify the existence of the various rotating domains of (111) oriented ZnTe, pole figure measurements were employed.…”

“…Although there was a large lattice mismatch between ZnTe and sapphire, the improvement of the ZnTe epilayer quality by the insertion of thin buffer layers between the substrate and the epilayer was confirmed [12,13]. A (111)-oriented ZnTe layer with single domain structure was formed on the c-plane sapphire substrate [12,14,18], while a (211)-oriented ZnTe layer was formed on the m-plane (10 10) substrate [13][14][15]. Hence, it was considered that the orientation of the ZnTe epilayer could be varied by using R-plane (10 14) and Splane (10 11) sapphire substrates.…”

“…These studies were mainly carried out by using X-ray diffraction (XRD) pole figure imaging analysis, which was revealed to be a powerful method for ZnTe epilayers on sapphire substrates [12][13][14][15][16][17][18]. The interface between ZnTe and sapphire were also evaluated by using the transmission electron microscope and the lattice mismatch accommodation and related strain distribution were analyzed [14,17,18].…”

“…Elemental sources of Zn (Osaka Asahi Metal, 6N super) and Te (Osaka Asahi Metal, 6N super) were used as source materials. Based on the optimized nucleation conditions employed for the layer grown on the c-plane sapphire, a 3.5-nm-thick ZnTe layer buffer was deposited at 100 ºC, and the annealing was performed at 300 °C for 10 min prior to the nucleation of ZnTe layers [12,14,18]. The growth of ZnTe layer was performed at 340 °C, and the growth rate and the film thickness were held constant at around 0.5 μm/hr and 1μm, respectively.…”

Growth and Crystal Orientation of ZnTe on m-Plane Sapphire with Nanofaceted Structure

“…Therefore, ZnTe layers have been prepared on sapphire substrates with various plane orientations to investigate their epitaxial relationship. [22][23][24][25][26][27][28] When m-plane sapphire was used, the ZnTe thin film showed (211)-plane orientation, 22 hence a larger EO effect was expected compared with (111)-plane-oriented film.…”